The present invention relates to an electronic device having a multiplicity of contact bumps that project from a housing and that are connected via rewiring configurations on an intermediate support and that also connect connecting lugs of the intermediate support to contact areas on a semiconductor chip.
Laterally directed mechanical stresses arise when a flat conductor bonding process is used to electrically and mechanically connect a semiconductor chip that is mounted on an organic intermediate support to conductive structures of the intermediate support material. These laterally directed mechanical stresses arise at a predetermined distance between the microscopically small contact areas on the semiconductor chip and the correspondingly dimensioned contact connecting lugs of the flat conductors of the intermediate support. This is because of the different coefficients of thermal expansion of the joining participants, namely the semiconductor chip and the plastic intermediate support. These laterally directed mechanical stresses, which can lead to cracking in the contact connecting lug up to a complete interruption of the connection, arise in the bonding bridge that includes an overhanging contact connecting lug. These laterally directed mechanical stresses increase with increasing distance between the bonded connection and the neutral point of the semiconductor.
Particularly in the case of semiconductor chips with a row of centrally positioned contact areas, the intermediate support material is open in the region of the contact areas, as a result of which a xe2x80x9cbonding channel windowxe2x80x9d in the intermediate support is kept free of support material. The etched-free conductor circuit pattern of the intermediate support is located in the bonding channel window above the contact areas of the semiconductor chip. During the flat conductor bonding process, these flat conductor structures are interrupted in a targeted manner and then are thermomechanically connected to the contact areas of the semiconductor chip. Since the region of the bonding channel window in the intermediate support is conventionally embodied as a slot with parallel edges, all of the flat conductor transitions to the contact windows are of the same length, irrespective of the size and extent of the semiconductor in the longitudinal direction. This means that at a large distance from the centrally located neutral point of the semiconductor chip, the risk of mechanical overloading during thermal cycling of the electronic device increases, the further away the bonded connection is from the neutral point of the semiconductor chip.
A conventional structure of an electronic device having a multiplicity of contact bumps which project on one side from a housing of the electronic device is shown in FIG. 3. The semiconductor chip 7 has a central row 13 of contact areas 6, only four of which are shown by way of example. The housing 15, whose contours are shown in FIG. 3, has an intermediate support 2 on which the multiplicity of contact bumps 1 is configured. The multiplicity of contact bumps 1 are connected via a multiplicity of flat conductors 3, only one of which is shown by way of example, to contact connecting lugs exposed in the bonding channel window 4 of the intermediate support 2 and to the contact areas 6 of the semiconductor chip 7 which are situated underneath at a fixed distance. In the conventional embodiment of the electronic device, the bonding channel window 4 has edges 10 and 11 that run parallel in the longitudinal direction. The contact connecting lugs 5 of the flat conductors 3 thus have the same lengths within the bonding channel window 4. Since these contact connecting lugs 5 are composed of flat conductor material, they are not round like the formerly customary bonding wires. These contact connecting lugs are thus at extreme risk of cracking if, because of the different expansion coefficients of the semiconductor chip 7 and of the intermediate support 2, the electronic device is exposed to thermal cycling. For bonded connections of this type, the further away a bonded connection, which includes a contact connecting lug of the intermediate support 2 and a contact area 6 of the semiconductor chip 7, is from the neutral point 8 of the semiconductor chip 7, the greater the risk of cracking during thermal cycling of the electronic device. This is because tensile stresses increase in the bonding bridge and load-relieving cracks occur.
It is accordingly an object of the invention to provide an electronic device which overcomes the above-mentioned disadvantageous of the prior art apparatus of this general type. In particular, it is an object of the invention to provide an electronic device that is constructed to reduce the risk that the bonded connections will crack or break, where these bonded connections include contact connecting lugs and semiconductor contact areas in a bonding channel window of the electronic device.
With the foregoing and other objects in view there is provided, in accordance with the invention, an electronic device, that includes a housing having an intermediate support formed with a bonding channel window having a center. A semiconductor chip has a plurality of contact areas disposed in the bonding channel window of the intermediate support. A plurality of contact bumps are configured on the intermediate support and project from the housing. A plurality of flat conductors have contact connecting lugs exposed in the bonding channel window of the intermediate support and connected to the plurality of the contact areas. The plurality of the flat conductors connect the plurality of the contact areas to the plurality of the contact bumps. The semiconductor chip has a neutral point disposed in the center of the bonding channel window of the intermediate support. The bonding channel window is formed with a width that increases with increasing distance from the neutral point of the semiconductor chip.
The neutral point of the semiconductor chip is configured in the center of the bonding channel window, and the width of the bonding channel window is greater with increasing distance from the neutral point. This electronic device has the advantage that the contact connecting lugs which are subjected to higher loading during thermal cycling of the electronic device, because they are further away from the neutral point of the semiconductor chip, are made longer and the tensile stress is thus reduced on both sides of the neutral axis of the contact connecting lugs. Because of the reduction of these tensile stresses during thermal cycling of the electronic device, the risk of cracking in the contact connecting lugs composed of flat conductor material is reduced to the extent that electronic devices constructed according to the invention withstand a higher degree of thermal cycling.
The bonded contact connecting lugs are usually exposed to extreme thermal cycling as early as in the production process, because the soldering bumps are not furnished until after the bonding process. Therefore, the reject rate when the contact bumps projecting from the housing are finally furnished can also be reduced by the manufactured electronic device. Consequently, the solution according to the invention increases both the productivity of the process for producing the electronic devices and the thermal-cycling resistance of the electronic devices to be supplied.
In accordance with an added feature of the invention, the edges of the bonding channel window are curved in the longitudinal direction, the radius of curvature being a multiple of the semiconductor chip length, preferably three to six times the length. This preferred embodiment advantageously realizes a continuously increasing width of the bonding channel window, with the result that the contact connecting lugs which are the furthest away from the central point of the semiconductor chip are made significantly longer than the contact connecting lugs in the vicinity of the neutral point.
In accordance with an additional feature of the invention, the bonding channel window is extended trapezoidally and symmetrically in its width in both longitudinal directions from the neutral point of the semiconductor chip. This embodiment has the advantage that it can be realized relatively simply since the width of the bonding channel window increases linearly with the length of the bonding channel window from the center.
In accordance with another feature of the invention, the bonding channel window increases from the neutral point up to the edge region of the semiconductor chip in the ratio of the length to half of the width of the intermediate support. This means, for example given a chip support width of 18 mm and a chip support length of 36 mm, that a central bonding channel window width of 0.5 mm in the center extends to 2 mm in the edge region. The contact connecting lug has to bridge about 250 xcexcm in the center and four times that, namely 1000 xcexcm, in the edge region, which increases its deformability and reduces its mechanical stresses during thermal cycling of the electronic device.
In accordance with a further feature of the invention, the contact bumps are configured in rows and columns on the intermediate support, preferably two to six contact bump rows are configured on both sides of the bonding channel window.
Electronic devices designed in this way can connect, with their contact bumps, a significantly higher number of contact areas of a semiconductor chip to, for example, associated pads on a circuit board than has been possible with a hitherto customary system support with lead frame for flat conductors.
In accordance with a further added feature of the invention, the contact areas of the semiconductor chip are configured in a row positioned centrally in the longitudinal direction on the semiconductor chip. This embodiment of the electronic device has the advantage that more favorable heat distributions can be achieved on the semiconductor chip since now active and passive components of the semiconductor chip which generate heat no longer have to be configured in the center of the semiconductor chip. Instead, they can be configured on the edges, at which a larger cooling area is available. The heat generation, which was conventionally concentrated in the center of the semiconductor chip, is thus no longer concentrated at the center of the semiconductor chip, but rather is distributed over the edge regions, thereby simultaneously reducing the internal stresses in the semiconductor chip.
The principle of a central bonding channel window having an increasing width for solving the above problem can also be applied to a plurality of bonding channel windows distributed in the longitudinal and transverse directions on the intermediate support. Therefore, in a preferred embodiment of the invention, a plurality of bonding channel windows are configured in the longitudinal and transverse directions in the intermediate support, the neutral point of the semiconductor chip being configured in a central bonding channel window. A structure of this type is used in the case where the distance between the contact bumps and the neutral point of the semiconductor chip becomes so large that, using just one bonding channel window, the loading caused by thermal cycling can no longer be compensated solely by widening a single bonding channel window. In the case of a plurality of bonding channel windows in the longitudinal and transverse directions of the intermediate support, the width of the bonding channel windows increases from the center to the edge in the longitudinal extent of the bonding channel windows. The increase is dependent on the ratio of the distance of the furthest contact bump to the distance of the nearest contact bump from the respective center of the bonding channel window.
In accordance with a further additional feature of the invention, the contact bumps are formed from solder balls disposed in predefined positions on the intermediate support after bonding the contact connecting lugs onto the contact areas of the semi-conductor chip. The solder balls are heated until they melt in the envisaged positions.
In accordance with a concomitant feature of the invention, the contact connecting lugs have a coating made of a gold alloy, while the contact area of the semiconductor chip is composed of an aluminum alloy. This embodiment has the advantage that during the bonding process, which is preferably carried out by ultrasonic excitation, the gold and aluminum alloys form a eutectic at low temperature, and consequently, a low ultrasonic energy suffices to raise the interface region between gold and aluminum to the eutectic melting point. On account of the flat conductor design of the contact connecting lugs, the frictional area between the contact area of the semiconductor chip and the area of the contact connecting lugs is larger than in the case of wire bonding. This ensures a significantly more stable bonded connection than in the case of conventional flat conductor technology using bonding wires.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a electronic device having a multiplicity of contact bumps, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.